/Engine/Source/Runtime/Engine/Public/SceneView.h
Transforms 2D screen coordinates into a 3D world-space origin and direction
static void DeprojectScreenToWorld ( const FVector2D & ScreenPos, const FIntRect & ViewRect, const FMatrix & InvViewMatrix, const FMatrix & InvProjMatrix, FVector & out_WorldOrigin, FVector & out_WorldDirection )